The present invention relates to a one-component adhesive based on a special epoxy resin system. The epoxy resin system according to the present invention makes it possible to furnish, without the use of components that are classified as toxic, irritating, or sensitizing and that would necessitate corresponding labeling according to European law, adhesives that are equivalent to conventional adhesive systems in terms of most utilization properties.
Epoxy-based resin systems have been successfully used for some time as adhesives or repair compounds for consumers, hobbyists, and craftsmen, and in the aviation, automotive, or electrical industry as adhesives, sealants, and for coating surfaces, or utilized as resin systems with a number of different materials for the manufacture of composite materials. Hardenable formulations that contain epoxy/hardener mixtures are particularly suitable as structural adhesives.
Epoxy-based resin systems are furthermore suitable as so-called structural foams for filling and reinforcing hollow components, in particular in automobile construction. Lightweight components for dimensionally consistent series production with high stiffness and structural strength are necessary for many areas of application. In vehicle construction in particular, because of the desire for weight saving, there is a great demand for lightweight components made of thin-walled structures that nevertheless possess sufficient stiffness and structural strength. One approach to achieving high stiffness and structural strength with the lowest possible component weight utilizes hollow parts that are produced from relatively thin sheet metal or plastic panels. Thin-walled metal sheets tend to deform easily, however. It has therefore been known for some time to foam out this cavity in hollow-body structures with a structural foam, which on the one hand prevents or minimizes deformation, and on the other hand enhances the strength and stiffness of these parts.
Foamed reinforcing and stiffening agents of this kind usually either are metal foams, or contain a thermally hardenable resin or binder such as, for example, epoxy resins. These compositions as a rule contain a blowing agent, fillers, and reinforcing fillers such as, for example, hollow microspheres made of glass. Such foams preferably have, in the foamed and cured state, a density from 0.3 to 0.7 g/cm3. After curing, these foams are said to withstand temperatures of more than 130° C., preferably more than 150° C., at least for a short time, without damage. Foamable, thermally hardenable compositions of this kind generally contain further constituents such as hardening agents, process adjuvants, stabilizers, dyes or pigments, optionally UV absorbers and adhesion-intensifying constituents.
One important property of the uncured resin system is its viscosity, which is significant in terms of handling and processability. Important material properties of the hardened formulations are, among others, adhesive power (often determined as tensile shear strength) and modulus of elasticity.
Reactive epoxy resin systems of the existing art often contain constituents that are classified as hazardous to water, toxic, irritating, and/or sensitizing and can result, for example, in contact dermatitis upon processing. According to European law, such preparations must be labeled with the corresponding hazard identifiers (e.g.: C, Xn, Xi, N). Preparations that contain reactive epoxy resins, and that do not require labeling under European law and exhibit reduced sensitizing potential, but that meet technical requirements in terms of processability and adhesive properties, are desirable for reasons of environmental protection, safety, and industrial hygiene.
It is known that reactive epoxy resins having a molar mass above 700 g/mol are not sensitizing and do not require labeling. Examples thereof are high-molecular-weight solids based on diglycidyl ethers of bisphenol A (DGEBA), and liquid epoxy-terminated polyethers having a high epoxy equivalent weight. These in some cases do not, however, exhibit the required viscosity properties for processing prior to curing, and/or the necessary strength properties after curing. High-molecular-weight solid epoxy novolacs have the high epoxy functionality (and correspondingly low epoxy equivalent weight) necessary for high strength, but because of their high viscosity at room temperature are not processable.